1. Field of the Invention
The present invention also relates to a semiconductor device including an oxide semiconductor.
In this specification, a “semiconductor device” refers to all devices which can function by utilizing semiconductor characteristics, and electro-optical devices, semiconductor circuits, electronic components, and electronic devices are all included in the category of the semiconductor device.
2. Description of the Related Art
In a semiconductor device with high withstand voltage for controlling large current, a so-called power device, silicon has been used mainly as a semiconductor material. However, it is said that the physical characteristics of a transistor using silicon reach the theoretical value limit, and a novel semiconductor material with which the characteristics can be improved has been demanded in order to realize a power device that has high withstand voltage and can control large current. As the semiconductor material that may improve the characteristics such as high withstand voltage, high conversion efficiency, or high-speed switching, for example, an oxide semiconductor has been attracted attention.
An oxide semiconductor has a wider band gap and lower intrinsic carrier density than silicon. In addition, the oxide semiconductor is advantageous in that it can be formed by a sputtering method or a wet method (such as a printing method) and has high mass productivity. In addition, a transistor is already known in which an oxide semiconductor is used for a semiconductor layer because a deposition temperature of the oxide semiconductor is as low as 300° C. to 500° C. (lower than or equal to the glass transition temperature, or approximately 700° C. at most) and the oxide semiconductor can be deposited over a glass substrate which is inexpensively available (Patent Document 1).